dt-bindings: watchdog: Add Mstar MSC313e WDT devicetree bindings documentation

This adds the documentation for the devicetree bindings of the Mstar
MSC313e watchdog driver, found from MSC313e SoCs and newer.

Signed-off-by: Romain Perier <romain.perier@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210611200801.52139-2-romain.perier@gmail.com


Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>